This invention relates to the measurement of the properties of subsurface earth formations surrounding a borehole and, more particularly, to an apparatus and method for measuring conductivity and certain properties of the formation invaded zone.
Induction logging has been employed for many years for measuring the conductivity of subsurface earth formations surrounding an earth borehole. In conventional induction logging a number of coils are wound on a mandrel. One or more transmitter coils are energized by an alternating current. The resultant oscillating magnetic field causes induction of currents in the formations which are substantially proportional to its conductivity. These currents, in turn, cause a voltage to be induced in receiver coils.
In the U.S. Pat. No. 3,090,910, there is disclosed a system for measuring the conductivity of formations surrounding a borehole using an induction technique that is different from conventional induction logging. A transmitter coil is energized with a sawtooth current, and the response induced in a spaced receiver coil is detected. The patent teaches that the rapidity of rise of the voltage induced in the receiver coil will vary in value inversely as the conductivity of the medium in the region that is being investigated; i.e., a slow rise time will be indicative of a relatively high conductivity, and a fast rise time will be indicative of a relatively low conductivity. The technique disclosed in the patent, referred to as "inductive probing", is intended to overcome certain limitations of conventional induction logging, for example, problems encountered due to skin effect phenomena and problems caused by measurements being affected by changes in the magnetic permeability of the medium being investigated.
It is an object of the present invention to provide another improved apparatus and method for measuring by induction the conductivity of earth formations surrounding a borehole, and also to provide a technique which is useful in determining properties of the invaded zone of said formations.